The present invention relates to rotary head type PCM recording/reproduction. More particularly, the invention concerns a rotary head type PCM recording/reproduction method and system suited for concealment and correction of such data errors or defects as produced by drop-outs of the recording medium, such as a magnetic tape, and the jamming of the head gap.
In the rotary head type PCM recording/reproducing apparatus (hereinafter also referred to simply as "R-DAT recorder"), the amount of data which can be recorded on a recording medium per unit area (i.e. recording density) is increased by a factor of about 10 (ten) when compared with other systems such as e.g. a stationary head type multi-track PCM recorder. When viewed from the standpoint of the requisite number of heads, the R-DAT recorder for which two heads are generally sufficient is more advantageous in respect to the manufacturing cost and reliability than the stationary head type multi-track PCM recorder which usually requires 10 to 20 heads. However, in the case of the rotary-head type PCM recorder, there may arise a problem in which no reproduced data is derived through one of the heads due to a drop-out of the magnetic tape serving as the recording medium, and a jamming of the head gap is caused by deposition of dust and/or magnetic particles scaled off from the tape, resulting in the amount of data to be reproduced being then decreased to a half to degrade significantly the quality of reproduction, such as the reproduced sound. In the worst case, the reproduction itself may become impossible.
In this regard, an examination in more detail will be made concerning the principle of recording/reproduction adopted in the hitherto known rotary head type PCM recorder as well as the problems inherent in such systems, with reference to FIGS. 1 to 6 of the accompanying drawings. FIG. 1 graphically illustrates two-channel stereophonic signals and the sampling points thereof. Referring to FIG. 1, the left-channel signal (represented by L) and the right-channel signal (represented by R) are sampled alternatively on a time-sequential basis in the order of L.sub.0 .fwdarw.R.sub.0 .fwdarw.L.sub.1 .fwdarw.R.sub.1 .fwdarw.L.sub.2 .fwdarw.R.sub.2 and so on. In other words, the two-channel signals are multiplexed on a time division basis so to be transmitted as a serial signal. FIG. 2 shows in a block diagram a general arrangement of a conventional rotary head type PCM recorder. In the figure, a reference numeral 1 denotes a left-channel signal input terminal, 2 denotes a right-channel signal input terminal, 3 denotes a left-channel input amplifier, 4 denotes a right-channel input amplifier, 5 denotes a multiplexer (MPX) for alternately changing over the channels to be recorded, 6 denotes an analog-to-digital (A/D) converter, 7 denotes a digital signal processing circuit for the recording signal, 8 denotes a recording output amplifier, 9 denotes a rotatable head cylinder, 10 denotes a magnetic head for a first channel (referred to also as "ch-1 head"), 11 denotes a magnetic head for a second channel (referred to also as "ch-2 head"), 12 denotes a magnetic tape, 13 denotes a reproducing preamplifier, 14 denotes a signal processing circuit for the reproduced signal, 15 denotes a digital-to-analog (D/A) converter, 16 denotes a demultiplexer (DMPX) for alternately changing over the reproduced channels, 17 denotes a left-channel output amplifier, 18 denotes a right-channel output amplifier, 19 denotes an output terminal for the left-channel signal, and numeral 20 denotes an output terminal for the right-channel signal. In the operation of this R-DAT recorder, the audio signals of the left-channel and right-channel are adjusted in level by the respective input amplifiers 3 and 4 to be subsequently subjected to channel multiplexing by the MPX 5 in the sequential order of L, R, L, R as shown in FIG. 1. The analog signal outputted from the MPX 5 is converted into a digital signal by the A/D converter 6 to be supplied to the digital signal processing circuit 7 where two basic processings are performed as mentioned below. First, the data transmitted thereto on a time serial basis is stored in a memory such as a RAM incorporated in the processing circuit, which data is then read-out from the memory in a sequence differing from the order in which the data has been written. This operation is referred to as the interleaving operation which is a sort of data dispersing processing for allowing the correct original signal to be restored even when the data as reproduced suffers from drop-outs or errors in the block. This interleaving processing is carried out by a data delay/rearraying circuit which is referred to as the interleaver. Secondly, the interleaved signal is so arrayed as to constitute error detection and correction blocks each of which has added thereto a code signal for error detection and correction such as a Reed-Solomon code. The signal processed as mentioned above is recorded on the magnetic tape 12 by the pair of rotary magnetic heads 10 and 11 after having been amplified through the recording amplifier 8. Upon reproduction, the signal picked up from the magnetic tape 12 by means of the magnetic heads 10 and 11 is amplified by the reproduction preamplifier 13 and undergoes, if necessary, correction of waveform referred to as waveform equalization, and is subsequently supplied to the digital signal processing circuit 14 of the reproduction system, where the input data is arrayed so as to constitute the data detection/correction blocks, the data suffering errors is detected and corrected (or concealed) and the sequence of the interleaved data is transformed to the original time-series sequence with the aid of a deinterleaver. This processing is referred to as the deinterleaving operation. The reproduced signal which has undergone the processings mentioned above is then converted to an analogue signal by the D/A converter 15 and subsequently is divided into the left-channel signal and right-channel signal by the demultiplexer circuit 16, resulting in the original audio signals L.sub.out and R.sub.out being produced at the respective output terminals 19 and 20. FIG. 3 shows a record pattern produced on a magnetic tape by the rotary head type PCM recorder, as viewed in the direction perpendicularly to the plane of the magnetic tape. In FIG. 3, a reference numeral 21 denotes tracks recorded by the magnetic head 10 for the first channel ch-1, and 22 denotes tracks recorded by the magnetic head 11 for the second channel ch-2. Because the heads 10 and 11 usually differ from each other is azimuth angle, it is possible to dispense with the non-recording zone (conventionally referred to as a guard band) normally required between the first channel ch-1 and the second channel ch-2. FIG. 4 shows a magnetic tape whose magnetic surface has a defect due to injury or deposition of dust particles. More specifically, a numeral 23 denotes a defect the record produced due to deposition of dust or the scaling off of magnetic particles from the tape. When the playback head passes by the defective areas where a drop-out or dust is present, the defects are reflected as errors in the reproduced signal. However, these errors are dispersed among data by virtue of the interleaving procedure described above, and can be corrected or concealed with the aid of the error correction code, so that the reproduced signal deinterleaved in succession to the error correction can be reproduced free of the influence of the drop-outs or defects. FIG. 5 of the accompanying drawings shows a typical one of the hitherto known record formats in which data is recorded in a block array through the interleaving procedure which is effective for error correction, as elucidated above. More specifically, FIG. 5 shows the record format of an amount of data which can be recorded and/or reproduced through a single scan by a single head. This capacity will be referred to as one field. As will be seen in FIG. 5, one field is composed of 256 blocks each of which is subdivided in the manner illustrated in FIG. 6. For particulars, reference may be made to Kentaro Odaka's article "A Rotary Head High Density Audio Tape Recorder" of "Technical Report Of Japan Electronic Communication Association", Vol. 1. 82, No. 190, EA 82-46 51, Nov. 30, 1982. Referring to FIG. 6, a numeral 24 designates a synchronizing signal SYNC (consisting of eight bits), 25 designates any identify code and block address code (consisting of 16 bits), 26 denotes a parity check code for data (consisting of 32 bits), 27 designates audio signal data (of 96 bits), and 28 designates a cyclic redundancy check (CRC) code (of 16 bits) for the error check operation. The parity code 26 is constituted by two parity words referred to as P parity and Q parity each of 16 bits. Further, data 27 is composed of six words W.sub.0, W.sub.1, W.sub.2, W.sub.3, W.sub.4 and W.sub.5 each of 16 bits. In this manner, one block is constituted by 168 bits in total, and 256 blocks in turn constitute one field. Referring again to FIG. 5, a format of interleaving will be described. It will be seen that any given block includes those words which are dispersed with a distance corresponding to 128 words. In other words, any two adjacent data words in a given block are chronographically distanced from each other for a time corresponding to 128 sampling points. By applying the interleaving procedure of this type, even such error which may take place over a succession of the blocks due to the drop-out of the tape or deposition of dust is dispersed among the blocks composed of the interleaved data words each dispersed by 128 sampling points and can be corrected by making use of the Reed-Solomon code and/or b-adjacent code as the error check code. Additionally, it will be noted that the even-numbered data such as L.sub.0, R.sub.0, L.sub.2, R.sub.2 and so forth (referred to also as even data) and the odd-numbered data such as L.sub.1, R.sub.1, L.sub.3, R.sub.3 and so forth (also referred to as odd data) are grouped in a left half and a right half, respectively, of one field, as viewed in FIG. 5. By virtue of the classification or grouping of the words in this manner, even when an error uncorrectable error takes place to such extent that a major part of the data contained in the preceding (left) half or alternatively in the succeeding (right) half of one field has been erased, the original analogue signal can be nevertheless reproduced approximately in the original form through a concealment procedure based on the previous value holding method or mean value interpolation method, so far as either the even data or odd data can be reproduced. In this connection, the length or extent of data over which the error concealment can be effected in this way is referred to as the error concealment length. In the case of the interleaving illustrated in FIG. 5, the error concealment length corresponds to 128 blocks.
As will be appreciated from the above discussion, the error correction or error concealment can be accomplished with a relatively high efficiency for the error produced due to the drop-out of the magnetic tape or deposition of dust thereon. The signal format of this type which is so arranged that the processing of all data contained in one format can be completed within a single scan period of one magnetic head will be referred to as the single-scan-completion type format or the single-field-completion type format. It should be however pointed out that the hitherto known format of the single-field-completion type suffers serious disadvantages in case the magnetic gap of one of the paired magnetic heads is jammed through deposition of dust or magnetic particles scaled off from the tape. Although error due to the jamming of the magnetic gap does not occur so frequently as burst error due to the drop-out, self-recovery from the jammed state can scarcely be expected. The jamming of the head occurring in the playback operation immediately brings about significant disturbance in the reproduced sound. Of course, the jamming occurring in the recording operation will give rise to a drop-out on the tape. With the hitherto known signal format, the signal can not completely be picked up by one magnetic head on the assumption that the head is rotated at 2000 r.p.m., with the result that the reproduced sound is intermittently intercepted at a time interval of 15 ms. Accordingly, the jamming of the head gap may provide a death-blow to the operation of PCM recording/reproducing apparatus which has been developed intrinsically with the objective of producing sound reproduction of a high fidelity. There exists thus a great demand for dealing successfully with this problem.
Concerning the present state of the R-DAT technology, reference may be made to the Japanese periodical "Electronics" of Japan Electronic Industries Association, Vol. 24, No. 10 (1984), p.p. 36-42, inter alia p.p. 40-42.